Speed control during motoring of a gas turbine engine

ABSTRACT

A system is provided for speed control during motoring of a gas turbine engine of an aircraft. The system includes an air turbine starter, a starter air valve operable to deliver compressed air to the air turbine starter, and a controller. The controller is operable to adjust the starter air valve to control motoring of the gas turbine engine based on measured feedback with lead compensation to reject disturbances attributable to dual to single or single to dual engine starting transitions.

BACKGROUND

This disclosure relates to gas turbine engines, and more particularly to systems and methods for controlling speed during motoring of one or more gas turbine engines.

Gas turbine engines are used in numerous applications, one of which is for providing thrust to an airplane. When the gas turbine engine of an airplane has been shut off for example, after an airplane has landed at an airport, the engine is hot and due to heat rise, the upper portions of the engine will be hotter than lower portions of the engine. When this occurs thermal expansion may cause deflection of components of the engine which may result in a “bowed rotor” condition. If a gas turbine engine is in such a bowed rotor condition it is undesirable to restart or start the engine.

One approach to mitigating a bowed rotor condition is to use a starter system to drive rotation of a spool within the engine for an extended period of time at a speed below which a resonance occurs (i.e., a critical speed or frequency) that may lead to damage when a sufficiently large bowed rotor condition is present. However, if a failure occurs in the starter system prior to completing bowed rotor mitigation or if a speed control of the spool cannot reliably regulate spool speed, the spool may accelerate until the critical speed is reached, potentially causing damage.

BRIEF DESCRIPTION

In an embodiment, a system is provided for speed control during motoring of a gas turbine engine of an aircraft. The system includes an air turbine starter, a starter air valve operable to deliver compressed air to the air turbine starter, and a controller. The controller is operable to adjust the starter air valve to control motoring of the gas turbine engine based on measured feedback with lead compensation to reject disturbances attributable to dual to single or single to dual engine starting transitions.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the compressed air is driven by an auxiliary power unit of the aircraft, a ground cart, or a cross engine bleed.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the measured feedback is based on an engine speed of a starting spool of the gas turbine engine.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the measured feedback is based on a starter speed of the air turbine starter.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the measured feedback is based on a starter air pressure.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the controller is a proportional-integral controller that outputs a duty cycle that a control signal generator converts into a valve command for the starter air valve.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the control signal generator is a pulse width modulator signal generator.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the controller adjusts the starter air valve based on a valve angle of the starter air valve.

According to an embodiment, a system of an aircraft includes an air turbine starter operable to drive rotation of a starting spool of a gas turbine engine of the aircraft in response to compressed air received from a compressed air source, a starter air valve operable to deliver the compressed air to the air turbine starter, and a controller. The controller is operable to adjust the starter air valve to control motoring of the gas turbine engine based on measured feedback with lead compensation to reject disturbances attributable to dual to single or single to dual engine starting transitions.

According to another embodiment, a method for speed control during motoring of a gas turbine engine is provided. The method includes commanding, by a controller, a starter air valve to control delivery of compressed air to an air turbine starter during motoring of the gas turbine engine. The starter air valve is adjusted based on measured feedback with lead compensation to reject disturbances attributable to dual to single or single to dual engine starting transitions.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the measured feedback is monitored based on one or more of: an engine speed of a starting spool of the gas turbine engine, a starter speed of the air turbine starter, and a starter air pressure

A technical effect of the apparatus, systems and methods is achieved by using a control sequence for bowed rotor mitigation of one or more gas turbine engines as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the present disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the present disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic illustration of an aircraft engine starting system in accordance with an embodiment of the disclosure;

FIG. 2 is another schematic illustration of an aircraft engine starting system in accordance with an embodiment of the disclosure;

FIG. 3 is a schematic illustration of a high spool gas path with a straddle-mounted spool in accordance with an embodiment of the disclosure;

FIG. 4 is a schematic illustration of a high spool gas path with an overhung spool in accordance with an embodiment of the disclosure;

FIG. 5 is a block diagram of a system for bowed rotor start mitigation in accordance with an embodiment of the disclosure;

FIG. 6 is a block diagram of a control loop for speed control during dry motoring in accordance with an embodiment of the disclosure; and

FIG. 7 is a flow chart illustrating a method in accordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

Various embodiments of the present disclosure are related to a bowed rotor start mitigation system in a gas turbine engine. Embodiments can include using a starter air valve to control a rotor speed of a starting spool of the gas turbine engine to mitigate a bowed rotor condition using a dry motoring process. During dry motoring, the starter air valve can be actively adjusted to deliver air pressure (i.e., compressed air) from an air supply to an air turbine starter of an engine starting system that controls starting spool rotor speed. Dry motoring may be performed by running an engine starting system at a lower speed with a longer duration than typically used for engine starting while dynamically adjusting the starter air valve to maintain the rotor speed and/or follow a dry motoring profile. Some embodiments increase the rotor speed of the starting spool to approach a critical rotor speed gradually and as thermal distortion is decreased they then accelerate beyond the critical rotor speed to complete the engine starting process. The critical rotor speed refers to a major resonance speed where, if the temperatures are unhomogenized, the combination of a bowed rotor and similarly bowed casing and the resonance would lead to high amplitude oscillation in the rotor and high rubbing of blade tips on one side of the rotor, especially in the high pressure compressor if the rotor is straddle-mounted.

A dry motoring profile for dry motoring can be selected based on various parameters, such as a modeled temperature value of the gas turbine engine used to estimate heat stored in the engine core when a start sequence is initiated and identify a risk of a bowed rotor. The modeled temperature value alone or in combination with other values (e.g., measured temperatures) can be used to calculate a bowed rotor risk parameter. For example, the modeled temperature can be adjusted relative to an ambient temperature when calculating the bowed rotor risk parameter. The bowed rotor risk parameter may be used to take a control action to mitigate the risk of starting the gas turbine engine with a bowed rotor. The control action can include dry motoring consistent with the dry motoring profile. In some embodiments, a targeted rotor speed profile of the dry motoring profile can be adjusted as dry motoring is performed.

A full authority digital engine control (FADEC) system or other system may send a message to the cockpit to inform the crew of an extended time start time due to bowed rotor mitigation actions prior to completing an engine start sequence. If the engine is in a ground test or in a test stand, a message can be sent to the test stand or cockpit based on the control-calculated risk of a bowed rotor. A test stand crew can be alerted regarding a requirement to keep the starting spool of the engine to a speed below the known resonance speed of the rotor in order to homogenize the temperature of the rotor and the casings about the rotor which also are distorted by temperature non-uniformity.

Referring now to FIG. 1, a schematic illustration of an aircraft 5 is depicted with a pair of engine systems 100A, 100B. Engine systems 100A, 100B include gas turbine engines 10A, 10B and engine starting systems 101A, 101B respectively. Engine systems 100A, 100B also include FADECs 102A, 102B to control gas turbine engines 10A, 10B and starting systems 101A, 101B. FADECs 102A, 102B may generally be referred to as controllers. FADECs 102A, 102B can communicate with respective engine control interfaces 105A, 105B using a digital communication bus 106. The engine control interfaces 105A, 105B can buffer engine system communication from aircraft level communication. Although depicted separately in FIG. 1, in some embodiments the engine control interfaces 105A, 105B are integrated with the FADECs 102A, 102B.

In an embodiment, the FADECs 102A, 102B and engine control interfaces 105A, 105B may each include memory to store instructions that are executed by one or more processors on one or more channels. The executable instructions may be stored or organized in any manner and at any level of abstraction, such as in connection with a controlling and/or monitoring operation of the gas turbine engines 10A, 10B of FIG. 1. The one or more processors can be any type of central processing unit (CPU), including a general purpose processor, a digital signal processor (DSP), a microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like. Also, in embodiments, the memory may include random access memory (RAM), read only memory (ROM), or other electronic, optical, magnetic, or any other computer readable medium onto which is stored data and control algorithms in a non-transitory form.

In the example of FIG. 1, an auxiliary power unit (APU) 113 and compressor 115 provide a compressed air source 114 to drive air turbine starters 120A, 120B of engine starting systems 101A, 101B. Compressed air from the compressed air source 114 is routed through ducts 117 and air starter valves 116A, 116B to the air turbine starters 120A, 120B. Various shutoff valves can also be included in ducts 117, such as a main shutoff valve 119 and engine shutoff valves 121A, 121B. One or more bleed valves 123 can be used to release compressed air from the ducts 117. According to an alternate embodiment, the compressed air source may be a ground cart, cross engine bleed or other source of air.

In embodiments, FADECs 102A, 102B can observe various engine parameters and starting system parameters to actively control dry motoring and prevent fault conditions from damaging the gas turbine engines 10A, 10B. For example, FADECs 102A, 102B can observe engine speeds (N2) of gas turbine engines 10A, 10B and may receive starter system parameters such as starter speeds (NS) and/or starter air pressures (SAP). In embodiments, FADECs 102A, 102B can adjust starter air valves 116A, 116B based on measured feedback with lead compensation to reject disturbances attributable to dual to single or single to dual engine starting transitions, such as switching from dry motoring both gas turbine engines 10A, 10B to dry motoring either gas turbine engine 10A or 10B.

In some cases, dry motoring can be performed simultaneously for engine systems 100A, 100B, where compressed air from the compressed air source 114 is provided to both air turbine starters 120A, 120B at the same time. When one of the engine systems 100A, 100B completes dry motoring before the other, a disturbance or pressure surge of compressed air may be experienced at the starter air valve 116A, 116B and air turbine starter 120A, 120B of the engine system 100A, 100B still performing dry motoring. The FADECs 102A, 102B can be configured with control laws to maintain a motoring speed below a threshold level (i.e., the critical rotor speed) for the engine system 100A, 100B while performing dry motoring based on compressed air source 114.

Although FIG. 1 depicts one example configuration, it will be understood that embodiments as described herein can cover a wide range of configurations, such as a four engine system. Further, the compressed air source 114 can include multiple sources other than APU 113 and compressor 115, such as a ground cart or cross engine bleed air.

Turning now to FIG. 2, a schematic of engine systems 100A, 100B and engine starting systems 101A, 101B for the gas turbine engines 10A, 10B of FIG. 1 are depicted according to an embodiment. In the example of FIG. 2, the digital communication bus 106 can include an aircraft, engine, and/or test stand communication bus to interface with FADECs 102A, 102B, engine control interfaces 105A, 105B, aircraft controls, e.g., a cockpit, various onboard computer systems, and/or a test stand (not depicted). Either or both channels of FADECs 102A, 102B can alternate on and off commands to respective electromechanical devices 110A, 110B coupled to starter air valves 116A, 116B to achieve a partially open position of the starter air valves 116A, 116B to control a flow of compressed air from compressed air source 114 (e.g., APU 113 and compressor 115 of FIG. 1) as a starter air flow to air turbine starters 120A, 120B during dry motoring. The air turbine starters 120A, 120B output torque to drive rotation of respective gas turbine engine shafts 50A, 50B of starting spools of the gas turbine engines 10A, 10B.

The FADECs 102A, 102B can monitor engine speed (N2), starter speed (NS), starter air pressure (SAP), and/or other engine parameters to determine an engine operating state and control the starter air valves 116A, 116B. Thus, the FADECs 102A, 102B can each establish a control loop with respect to a motoring speed (N2 and/or NS) and/or starter air pressure to adjust positioning of the starter air valves 116A, 116B. In some embodiments, the starter air valves 116A, 116B are discrete valves designed as on/off valves that are typically commanded to either fully opened or fully closed. However, there is a time lag to achieve the fully open position and the fully closed position. By selectively alternating an on-command time with an off-command time through the electromechanical devices 110A, 110B, intermediate positioning states (i.e., partially opened/closed) can be achieved. The FADECs 102A, 102B can modulate the on and off commands (e.g., as a duty cycle using pulse width modulation) to the electromechanical devices 110A, 110B to further open the starter air valves 116A, 116B and increase a rotational speed of the gas turbine engine shafts 50A, 50B. In an embodiment, the electromechanical devices 110A, 110B have a cycle time defined between an off-command to an on-command to the off-command that is at most half of a movement time for the starter air valves 116A, 116B to transition from fully closed to fully open. Pneumatic lines or mechanical linkage (not depicted) can be used to drive the starter air valves 116A, 116B between the open position and the closed position. The electromechanical devices 110A, 110B can each be a solenoid that positions the starter air valves 116A, 116B based on intermittently supplied electric power as commanded by the FADECs 102A, 102B. In an alternate embodiment, the electromechanical devices 110A, 110B are electric valves controlling muscle air to adjust the position of the starter air valves 116A, 116B as commanded by the FADECs 102A, 102B.

In an alternate embodiment, rather than using electromechanical devices 110A, 110B to achieve a partially open position of the starter air valves 116A, 116B, the starter air valves 116A, 116B can be variable position valves that are dynamically adjustable to selected valve angles by the FADECs 102A, 102B. When implemented as variable position valves, the starter air valves 116A, 116B can be continuous/infinitely adjustable and hold a commanded valve angle, which may be expressed in terms of a percentage open/closed and/or an angular value (e.g., degrees or radians). Performance parameters of the starter air valves 116A, 116B can be selected to meet dynamic response requirements. For example, in some embodiments, the starter air valves 116A, 116B each have a response rate of 0% to 100% open in less than 40 seconds. In other embodiments, the starter air valves 116A, 116B each have a response rate of 0% to 100% open in less than 30 seconds. In further embodiments, the starter air valves 116A, 116B each have a response rate of 0% to 100% open in less than 20 seconds.

In some embodiments, the FADECs 102A, 102B can each monitor a valve angle of the starter air valves 116A, 116B when valve angle feedback is available. The FADECs 102A, 102B can establish an outer control loop with respect to motoring speed and an inner control loop with respect to the valve angle of the starter air valves 116A, 116B.

FIGS. 3 and 4 depict two example engine configurations of the gas turbine engines 10A, 10B of FIG. 1. FIG. 3 is an example of a straddle-mounted spool 32A as a starting spool configuration. This configuration places two bearing compartments 37A and 39A (which may include a ball bearing and a roller bearing respectively), outside of the plane of most of the compressor disks of high pressure compressor 52A and at outside at least one of the turbine disks of high pressure turbine 54A. In contrast with a straddle-mounted spool arrangement, other embodiments may be implemented using an over-hung mounted spool 32B as depicted in FIG. 4 as a starting spool configuration. In over-hung mounted spool 32B, a bearing compartment 37B is located forward of the first turbine disk of high pressure turbine 54B such that the high pressure turbine 54B is overhung, and it is physically located aft of its main supporting structure. The use of straddle-mounted spools has advantages and disadvantages in the design of a gas turbine, but one characteristic of the straddle-mounted design is that the span between the bearing compartments 37A and 39A is long, making the amplitude of the high spot of a bowed rotor greater and the resonance speed that cannot be transited prior to temperature homogenization is lower. For any thrust rating, the straddle mounted arrangement, such as straddle-mounted spool 32A, gives Lsupport/Dhpt values that are higher, and the overhung mounted arrangement, such as overhung spool 32B, can be as much as 60% of the straddle-mounted Lsupport/Dhpt. Lsupport is the distance between bearings (e.g., between bearing compartments 37A and 39A or between bearing compartments 37B and 39B), and Dhpt is the diameter of the last blade of the high pressure turbine (e.g., high pressure turbine 54A or high pressure turbine 54B). As one example, a straddle-mounted engine starting spool, such as straddle-mounted spool 32A, with a roller bearing at bearing compartment 39A located aft of the high pressure turbine 54A may be more vulnerable to bowed rotor problems since the Lsupport/Dhpt ranges from 1.9 to 5.6.

FIGS. 3 and 4 also illustrate an air turbine starter 120 (e.g., air turbine starter 120A or 120B of FIGS. 1 and 2) interfacing through gearbox 124 via a tower shaft 55 with the straddle-mounted spool 32A proximate high compressor 52A and interfacing via tower shaft 55 with the overhung mounted spool 32B proximate high compressor 52B as part of a starting system. The straddle-mounted spool 32A and the over-hung mounted spool 32B are both examples of a starter spool having a gas turbine engine shaft 50 driven by the air turbine starter 120, such as gas turbine engine shafts 50A, 50B driven by air turbine starters 120A, 120B of FIG. 2.

FIG. 5 is a block diagram of a system 200 for bowed rotor start mitigation that may control either of the starter air valves 116A, 116B of FIGS. 1 and 2 via control signals 210 in accordance with an embodiment. The system 200 may also be referred to as a bowed rotor start mitigation system. In the example of FIG. 5, the system 200 includes an onboard model 202 operable to produce a compressor exit temperature T₃ and a compressor inlet flow W₂₅ of one of the gas turbine engines 10A, 10B of FIG. 1 for use by a core temperature model 204. The onboard model 202 is configured to synthesize or predict major temperatures and pressures throughout one of the gas turbine engines 10A, 10B of FIG. 1 beyond those sensed by sensors positioned about the gas turbine engines 10A, 10B. The onboard model 202 and core temperature model 204 are examples of a first thermal model and a second thermal model that may be separately implemented or combined as part of a controller 102 (e.g., FADECs 102A, 102B of FIG. 1).

Engine parameter synthesis is performed by the onboard model 202, and the engine parameter synthesis may be performed using the technologies described in U.S. Patent Publication No. 2011/0077783, the entire contents of which are incorporated herein by reference thereto. Of the many parameters synthesized by onboard model 202 at least two are outputted to the core temperature model 204, T₃, which is the compressor exit gas temperature of each gas turbine engine 10A, 10B and W₂₅, which is the air flow through the compressor. Each of these values are synthesized by onboard model 202 and inputted into the core temperature model 204 that synthesizes or provides a heat state (T_(core)) of each gas turbine engine 10A, 10B. T_(core) can be determined by a first order lag or function of T₃ and a numerical value X (e.g., f(T₃,X)), wherein X is a value determined from a lookup table stored in memory of controller 102. Accordingly, X is dependent upon the synthesized value of W₂₅. In other words, W₂₅ when compared to a lookup table of the core temperature model 204 will determine a value X to be used in determining the heat state or T_(core) of each gas turbine engine 10A, 10B. In one embodiment, the higher the value of W₂₅ or the higher the flow rate through the compressor the lower the value of X.

The heat state of each engine 10A, 10B during use or T_(core) is determined or synthesized by the core temperature model 204 as each engine 10A, 10B is being run. In addition, T₃ and W₂₅ are determined or synthesized by the onboard model 202 and/or the controller 102 as each engine 10A, 10B is being operated.

At engine shutdown, the current or most recently determined heat state of the engine or T_(core shutdown) of an engine 10A, 10B is recorded into data storage unit (DSU) 104, and the time of the engine shutdown t_(shutdown) is recorded into the DSU 104. The DSU 104 retains data between shutdowns using non-volatile memory. Each engine 10A, 10B may have a separate DSU 104. Time values and other parameters may be received on digital communication bus 106. As long as electrical power is present for the controller 102 and DSU 104, additional values of temperature data may be monitored for comparison with modeled temperature data to validate one or more temperature models (e.g., onboard model 202 and/or core temperature model 204) of each gas turbine engine 10A, 10B.

During an engine start sequence or restart sequence, a bowed rotor start risk model 206 (also referred to as risk model 206) of the controller 102 is provided with the data stored in the DSU 104, namely T_(core shutdown) and the time of the engine shutdown t_(shutdown). In addition, the bowed rotor start risk model 206 is also provided with the time of engine start t_(start) and the ambient temperature of the air provided to the inlet of each engine 10A, 10B T_(inlet) or T₂. T₂ is a sensed value as opposed to the synthesized value of T₃.

The bowed rotor start risk model 206 maps core temperature model data with time data and ambient temperature data to establish a motoring time t_(motoring) as an estimated period of motoring to mitigate a bowed rotor of each gas turbine engine 10A, 10B. The motoring time t_(motoring) is indicative of a bowed rotor risk parameter computed by the bowed rotor start risk model 206. For example, a higher risk of a bowed rotor may result in a longer duration of dry motoring to reduce a temperature gradient prior to starting each gas turbine engine 10A, 10B of FIG. 1. In one embodiment, an engine start sequence may automatically include a modified start sequence; however, the duration of the modified start sequence prior to a normal start sequence will vary based upon the time period t_(motoring) that is calculated by the bowed rotor start risk model 206. The motoring time t_(motoring) for predetermined target speed N_(target) of each engine 10A, 10B is calculated as a function of T_(core shutdown), t_(shutdown), t_(start) and T₂, (e.g., f(T_(core shutdown), t_(shutdown), t_(start) and T₂) while a target speed N_(target), is a predetermined speed that can be fixed or vary within a predetermined speed range of N_(targetMin) to N_(targetMax). In other words, the target speed N_(target) may be the same regardless of the calculated time period t_(motoring) or may vary within the predetermined speed range of N_(targetMin) to N_(targetMax). The target speed N_(target) may also be referred to as a dry motoring mode speed.

Based upon these values (T_(core shudown), t_(shutdown), t_(start) and T₂) the motoring time t_(motoring) at a predetermined target speed N_(target) for the modified start sequence of each engine 10A, 10B is determined by the bowed rotor start risk model 206. Based upon the calculated time period t_(motoring) which is calculated as a time to run each engine 10A, 10B at a predetermined target speed N_(target) in order to clear a “bowed condition”. In accordance with an embodiment of the disclosure, the controller 102 can run through a modified start sequence upon a start command given to each engine 10A, 10B by an operator of the engines 10A, 10B, such as a pilot of an airplane the engines 10A, 10B are used with. It is understood that the motoring time t_(motoring) of the modified start sequence may be in a range of 0 seconds to minutes, which depends on the values of T_(core shutdown), t_(shutdown), t_(start) and T₂.

In an alternate embodiment, the modified start sequence may only be run when the bowed rotor start risk model 206 has determined that the motoring time t_(motoring) is greater than zero seconds upon receipt of a start command given to each engine 10A, 10B. In this embodiment and if the bowed rotor start risk model 206 has determined that t_(motoring) is not greater than zero seconds, a normal start sequence will be initiated upon receipt of a start command to each engine 10A, 10B.

Accordingly and during an engine command start, the bowed rotor start risk model 206 of the system 200 may be referenced wherein the bowed rotor start risk model 206 correlates the elapsed time since the last engine shutdown time and the shutdown heat state of each engine 10A, 10B as well as the current start time t_(start) and the inlet air temperature T₂ in order to determine the duration of the modified start sequence wherein motoring of each engine 10A, 10B at a reduced speed N_(target) without fuel and ignition is required. As used herein, motoring of each engine 10A, 10B in a modified start sequence refers to the turning of a starting spool by air turbine starter 120A, 120B at a reduced speed N_(target) without introduction of fuel and an ignition source in order to cool the engine 10A, 10B to a point wherein a normal start sequence can be implemented without starting the engine 10A, 10B in a bowed rotor state. In other words, cool or ambient air is drawn into the engine 10A, 10B while motoring the engine 10A, 10B at a reduced speed in order to clear the “bowed rotor” condition, which is referred to as a dry motoring mode.

The bowed rotor start risk model 206 can output the motoring time t_(motoring) to a motoring controller 208. The motoring controller 208 uses a dynamic control calculation in order to determine a required valve position of the starter air valve 116A, 116B used to supply an air supply or compressed air source 114 to the engine 10A, 10B in order to limit the motoring speed of the engine 10A, 10B to the target speed N_(target) due to the position of the starter air valve 116A, 116B. The required valve position of the starter air valve 116A, 116B can be determined based upon an air supply pressure as well as other factors including but not limited to ambient air temperature, parasitic drag on the engine 10A, 10B from a variety of engine driven components such as electric generators and hydraulic pumps, and other variables such that the motoring controller 208 closes the loop for an engine motoring speed target N_(target) for the required amount of time based on the output of the bowed rotor start risk model 206. In one embodiment, the dynamic control of the valve position (e.g., open state of the valve (e.g., fully open, ½ open, ¼ open, etc.) in order to limit the motoring speed of the engine 10A, 10B) is controlled via duty cycle control (on/off timing using pulse width modulation) of electromechanical device 110A, 110B for starter air valves 116A, 116B.

When variable position starter air valves are used as the starter air valves 116A, 116B, a valve angle 207 can be provided to motoring control 208 based on valve angle feedback. A rotor speed N2 can be provided to the motoring controller 208 and a mitigation monitor 214, where motoring controller 208 and a mitigation monitor 214 may be part of controller 102.

The risk model 206 can determine a bowed rotor risk parameter that is based on the heat stored (T_(core)) using a mapping function or lookup table. When not implemented as a fixed rotor speed, the bowed rotor risk parameter can have an associated dry motoring profile defining a target rotor speed profile over an anticipated amount of time for the motoring controller 208 to send control signals 210, such as valve control signals for controlling starter air valves 116A, 116B of FIG. 3.

The bowed rotor risk parameter may be quantified according to a profile curve selected from a family of curves that align with observed aircraft/engine conditions that impact turbine bore temperature and the resulting bowed rotor risk. In some embodiments, an anticipated amount of dry motoring time can be used to determine a target rotor speed profile in a dry motoring profile for the currently observed conditions. As one example, one or more baseline characteristic curves for the target rotor speed profile can be defined in tables or according to functions that may be rescaled to align with the observed conditions.

In summary with reference to FIG. 5, as one example of an aircraft that includes systems as described herein, onboard model 202 and core temperature model 204 may run on controller 102 of the aircraft to track heat stored (T_(ore)) in the turbine at the time of engine shutdown. Modeling of potential heat stored in the system may be performed as a turbine disk metal temperature model in the core temperature model 204. When the aircraft lands, engines typically operate at idle for a cool down period of time, e.g., while taxiing to a final destination. When an engine shutdown is detected, model state data can be logged by the DSU 104 prior to depowering. When the controller 102 powers on at a later time and model state data can be retrieved from the DSU 104, and the bowed rotor start risk model 206 can be updated to account for the elapsed time. When an engine start is requested, a bowed rotor risk can be assessed with respect to the bowed rotor start risk model 206. Extended dry motoring can be performed during an engine start process until the bow risk has sufficiently diminished.

In reference to FIG. 5, the mitigation monitor 214 can operate in response to receiving a complete indicator 212 to run a verification of the bowed rotor mitigation. The mitigation monitor 214 can provide mitigation results 216 to the motoring controller 208 and may provide result metrics 218 to other systems, such a maintenance request or indicator. The mitigation monitor 214 may also run while dry motoring is active to determine whether adjustments to the dry motoring profile are needed. If the mitigation monitor 214 determines that a bowed rotor condition still exists, the motoring controller 208 may restart dry motoring, or a maintenance request or indicator can be triggered along with providing result metrics 218 for further analysis. Metrics of attempted bowed rotor mitigation can be recorded in the DSU 104 based on determining that the attempted bowed rotor mitigation was unsuccessful or incomplete.

FIG. 6 is a block diagram of a control loop 250 for speed control during dry motoring in accordance with an embodiment. The control loop 250 may be part of the motoring controller 208 of FIG. 5. In the example of FIG. 6, the control loop 250 includes a proportional-integral controller 252 that can determine a duty cycle 254 based on an error signal 256 as the difference between a desired speed 258 and a lead feedback 260 from a lead compensator 262. The lead compensator 262 can determine the lead feedback 260 based on measured feedback 264, such as engine speed (N2), starter speed (NS), and/or starter air pressure (SAP). The lead compensator 262 may implement a transfer function that provides a phase lead (i.e., a positive phase angle) in its frequency response to increase system stability and speed of response, which can be determined, for example, through system modeling according to known modeling techniques. In some embodiments, the duty cycle 254 is provided to a control signal generator 266 that outputs a valve command 268, such as pulse width modulation generation for energizing/de-energizing starter valves 116A, 116B. In alternate embodiments, the control signal generator 266 is adjusted based on a valve angle of the starter air valve 116A, 116B.

FIG. 7 is a flow chart illustrating a method 300 of speed control during motoring of the gas turbine engines 10A, 10B of FIG. 1 in accordance with an embodiment. The method 300 of FIG. 7 is described in reference to FIGS. 1-6 and may be performed with an alternate order and include additional steps. Before initiating bowed rotor start mitigation, a bowed rotor determination step can be performed to estimate a need for bowed rotor start mitigation. Examples include the use of models and/or stored/observed engine/aircraft state data, such as data received from DSU 104, digital communication bus 106, and/or reading data from one or more temperature sensors of the gas turbine engines 10A, 10B.

At block 302, the controller 102 (e.g., FADEC 102A, 102B) commands a starter air valve (e.g., starter air valves 116A, 116B) to control delivery of compressed air to an air turbine starter (e.g., air turbine starters 120A, 120B) during motoring of either or both of the gas turbine engines 10A, 10B. At block 304, the controller 102 adjusts the starter air valve 116A, 116B based on measured feedback with lead compensation to reject disturbances attributable to dual to single or single to dual engine starting transitions. At block 306, the controller 102 monitors the measured feedback based on one or more of: an engine speed (N2) of a starting spool of the gas turbine engine 10A, 10B, a starter speed (NS) of the air turbine starter, and a starter air pressure (SAP). The compressed air can be driven by an auxiliary power unit 113 of the aircraft 5 or other source such as a ground cart or cross engine bleed. The controller 102 can include a proportional-integral controller that outputs a duty cycle that a control signal generator converts into a valve command for the starter air valve 116A, 116B. The control signal generator can be a pulse width modulator signal generator. The controller 102 adjusts the starter air valve 116A, 116B based on a valve angle of the starter air valve 116A, 116B.

Accordingly and as mentioned above, it is desirable to detect, prevent and/or clear a “bowed rotor” condition in a gas turbine engine that may occur after the engine has been shut down. As described herein and in one non-limiting embodiment, the FADECs 102A, 102B (e.g., controller 102) may be programmed to automatically take the necessary measures in order to provide for a modified start sequence without pilot intervention other than the initial start request. In an exemplary embodiment, the FADECs 102A, 102B, DSU 104 and/or engine control interfaces 105A, 105B comprises a microprocessor, microcontroller or other equivalent processing device capable of executing commands of computer readable data or program for executing a control algorithm and/or algorithms that control the start sequence of the gas turbine engine. In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the execution of Fourier analysis algorithm(s), the control processes prescribed herein, and the like), the FADECs 102A, 102B, DSU 104 and/or engine control interfaces 105A, 105B may include, but not be limited to, a processor(s), computer(s), memory, storage, register(s), timing, interrupt(s), communication interfaces, and input/output signal interfaces, as well as combinations comprising at least one of the foregoing. For example, the FADECs 102A, 102B, DSU 104 and/or engine control interfaces 105A, 105B may include input signal filtering to enable accurate sampling and conversion or acquisitions of such signals from communications interfaces. As described above, exemplary embodiments of the disclosure can be implemented through computer-implemented processes and apparatuses for practicing those processes.

While the present disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the present disclosure is not limited to such disclosed embodiments. Rather, the present disclosure can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the present disclosure. Additionally, while various embodiments of the present disclosure have been described, it is to be understood that aspects of the present disclosure may include only some of the described embodiments. Accordingly, the present disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

The invention claimed is:
 1. A system for speed control during motoring of a first gas turbine engine of an aircraft comprising at least two gas turbine engines, the system comprising: an air turbine starter; a starter air valve operable to deliver compressed air to the air turbine starter; and a controller operable to adjust the starter air valve to control motoring of the first gas turbine engine based on measured feedback with lead compensation providing a phase lead with a positive phase angle frequency response to reject disturbances attributable to dual to single or single to dual engine starting transitions between the at least two gas turbine engines.
 2. The system as in claim 1, wherein the compressed air is driven by an auxiliary power unit of the aircraft, a ground cart, or a cross engine bleed.
 3. The system as in claim 1, wherein the measured feedback is based on an engine speed of a starting spool of the first gas turbine engine.
 4. The system as in claim 1, wherein the measured feedback is based on a starter speed of the air turbine starter.
 5. The system as in claim 1, wherein the measured feedback is based on a starter air pressure.
 6. The system as is claim 1, wherein the controller is a proportional-integral controller that outputs a duty cycle that a control signal generator converts into a valve command for the starter air valve.
 7. The system as in claim 6, wherein the control signal generator is a pulse width modulator signal generator.
 8. The system as in claim 1, wherein the controller adjusts the starter air valve based on a valve angle of the starter air valve.
 9. A system of an aircraft, the system comprising: an air turbine starter operable to drive rotation of a starting spool of a first gas turbine engine of the aircraft in response to compressed air received from a compressed air source, wherein the aircraft comprises at least two gas turbine engines; a starter air valve operable to deliver the compressed air to the air turbine starter; and a controller operable to adjust the starter air valve to control motoring of the gas turbine engine based on measured feedback with lead compensation providing a phase lead with a positive phase angle frequency response to reject disturbances attributable to dual to single or single to dual engine starting transitions between the at least two gas turbine engines.
 10. The system of claim 9, wherein the compressed air source comprises an auxiliary power unit of the aircraft, a ground cart, or a cross engine bleed.
 11. The system of claim 9, wherein the measured feedback is based on one or more of: an engine speed of the starting spool of the first gas turbine engine, a starter speed of the air turbine starter, and a starter air pressure.
 12. The system of claim 9, wherein the controller is a proportional-integral controller that outputs a duty cycle that a control signal generator converts into a valve command for the starter air valve.
 13. The system of claim 12, wherein the control signal generator is a pulse width modulator signal generator.
 14. The system of claim 9, wherein the controller adjusts the starter air valve based on a valve angle of the starter air valve.
 15. A method for speed control during motoring of a first gas turbine engine of an aircraft comprising at least two gas turbine engines, the method comprising: commanding, by a controller, a starter air valve to control delivery of compressed air to an air turbine starter during motoring of the first gas turbine engine; and adjusting the starter air valve based on measured feedback with lead compensation providing a phase lead with a positive phase angle frequency response to reject disturbances attributable to dual to single or single to dual engine starting transitions between the at least two gas turbine engines.
 16. The method as in claim 15, wherein the compressed air source comprises an auxiliary power unit of the aircraft, a ground cart, or a cross engine bleed.
 17. The method as in claim 15, wherein the measured feedback is monitored based on one or more of: an engine speed of a starting spool of the first gas turbine engine, a starter speed of the air turbine starter, and a starter air pressure.
 18. The method as in claim 15, wherein the controller is a proportional-integral controller that outputs a duty cycle that a control signal generator converts into a valve command for the starter air valve.
 19. The method as in claim 18, wherein the control signal generator is a pulse width modulator signal generator.
 20. The method as in claim 15, wherein the controller adjusts the starter air valve based on a valve angle of the starter air valve. 